All awards shows are popularity contests to some degree. And that's fine!
Does game sales matter? Yes of course. Does review scores? Definitely! What about the mainstream status of a game? Yes!
Even if you compare games to movies, it's much more difficult for more obscure games to get enough attention to be nominated. In stuff like Bafta film awards the people deciding the nominations may say even over a hundred movies over the course of a few weeks or months. A movie is generally a 90—180 minute investment.
In comparison, one game takes generally 10-100 hours, especially if you play them until the end. It is simply not possibly for reviewers to play every game released this year, not even all of the games that might deserve awards.
So then, the game's mainstream status does matter in terms of what gets nominated. Because not every outlet is able to play everything. Did Case of the Golden Idol deserve a nomination? Perhaps, but it was never going to get it. That said, saying only sales and mainstream status matters is too reductive. They matter a big deal, but they aren't the only thing. Call of Duty MW3 was never going to be an awards darling for a reason.